Preheat the oven to 375°F (190°C). Boil salted water and par-cook red potatoes for 3-4 minutes, then drain.

Slice eggplant, zucchini, yellow squash, and bell pepper into ¼-inch slices.

Melt butter in a saucepan, whisk in flour, then gradually add warm milk. Cook until thickened, season with salt and pepper.

Spread tomato sauce in a baking dish, mixing in minced garlic, thyme, and olive oil.

Layer sliced vegetables starting with eggplant, followed by zucchini, yellow squash, and bell pepper. Drizzle with olive oil and season.

Pour Béchamel sauce over the layers, cover with parchment, and bake for 55-60 minutes.

Cool for 5 minutes before serving. Enjoy with crusty bread or over pasta.